An anti-theft device for a motor vehicle, of the type comprising:                a stator,        a rotor mounted to rotate in the stator between a first extreme angular position known as the “off” position and at least one second angular position, and        means for interrupting the return travel of the rotor from the second position to the “off” position in a position known as the “non-return” position interpolated angularly between the second position and “off” position, these interrupt means being electrically deactivatableis already known in the state of the art.        
An anti-theft device of electronic type advantageously has the above characteristics.
An electronic anti-theft device generally comprises a control knob, which rotates as one with the rotor, forming a member for manually controlling the starting of the engine of the vehicle and for locking the vehicle steering column.
With a view to allowing the steering column to be unlocked, an electronic anti-theft device comprises an electronic identification device particularly consisting of an electronic identifier worn or carried by an authorized user of the vehicle, and a unit for identifying this authorized user, carried on board the vehicle. The electronic identifier may, for example, have the appearance of a key or of a badge.
With a view to initiating the starting of the engine, the electronic anti-theft device comprises a multi-position electric rotary switch that rotates as one with the rotor. This rotary switch allows the starting of the engine to be controlled. What happens is that the control knob can be placed successively in a first extreme angular position of rest known as the “off” position, in an intermediate angular position known as the “on” position, and in a second extreme angular position, at the opposite extreme to the first extreme position, known as the “start” position. Starting out from the “off” position, the angular travel of the control knob is, for example, about 90° to reach the “on” position and about 130° to reach the “start” position.
It will be noted that, in general, the locking of the steering column is rendered possible only when the control knob and the rotor are in the “off” position.
Certain regulations dictate that, in a vehicle of the type having an automatic gearbox, return of the control knob (which rotates as one with the rotor) from its “on” position to its “off” position be prevented while the gear selector has not been returned to the “park” position. The user is thus alerted to this situation.
The means of interrupting the return travel of the rotor allow this regulation to be complied with by allowing the rotor to be immobilized in the “non-return” position which is situated, for example, 45° away from the “off” position.
The means of interrupting the return travel of the rotor generally comprise an electromagnet, borne by the stator, equipped with a rod (plunger) that can be moved between a projecting position, in which this rod can engage with the rotor in order to immobilize it, and a retracted position in which this rod is clear of the rotor. The rod is returned elastically to its projecting position by mechanical means. The rod is urged towards its retracted position, against its return force, by applying power to the electromagnet. When the electromagnet is not powered the rod is therefore returned elastically to the projecting position.
However, when the electromagnet is not powered, the rotor is immobilized by the rod of this electromagnet only if this rod is aligned with a corresponding orifice for accommodating the rod, formed in the rotor.
Thus, the rotor usually comprises a “non-return” orifice into which the rod of the electromagnet automatically fits when the rotor is in the non-return position and the automatic gearbox selector is not in its “park” position.
The rotor may also comprise another orifice into which the rod of the electromagnet fits when the rotor is in the “off” position and an authorized user has not been recognized by the identification device, so as to lock the rotor in this “off” position.
Through a quest for standardization by limiting the number of types of anti-theft device, an electronic anti-theft device comprising the function described hereinabove for locking the rotor in the “non-return” position may be fitted to a vehicle equipped with a non-automatic gearbox. In this case, this function is not used.
Nonetheless, whether or not the vehicle to which the anti-theft device is fitted is equipped with an automatic gearbox, if there is an electrical failure preventing the electromagnet from being powered, when the user moves the control knob from its “on” position to its “off” position, the rod of the electromagnet, returned elastically to the projecting position, fits automatically into the “non-return” orifice of the rotor so as to lock the control knob and the rotor in the “non-return” position.
Now, certain regulations demand that, in spite of the above electrical failure and the fact that it is impossible to power the electromagnet in the normal way, it must be possible for the control knob to be returned to the “off” position in order to be able to lock the steering column and protect the vehicle against theft.